Hydrogeomorphological studies in the Trichirappalli environs,Tamil Nadu, India using Remote Sensing technology
Data used
IRS 1A LISS I (Path 24 and Row 61) imagery for visual interpretation
Survey of India toposheets Nos.56J/9,10,11,13,14,15,12&16 and 58N/1,2,3,4 and 80/1.
Methodology:
The IRS IA LISS I (false colour composite) has been visually interpreted by using standard interpretation keys such as colour, tone, texture, pattern of drainage, shape and topography etc., to prepare geomorphological maps. All the conventional information such as geological, hydrogeological, well inventory data and also the information collected during the field checks was used in the finalization of the hydro-geomorphological maps.
Results and discussion:
Geologically, Trichirappalli environs falls under both hard crystalline and sedimentary formations. The hard rocks include hornblende biotite gneisses, charnockites, granitic gneisses and anorthosites of Archaean age. The pegmatite / quartzite's vein is present in the intrusive bodies, which act as good groundwater barriers to store groundwater (Karanth et.al. 1992; Scuba Rao, 1992). A number of structural features like antifoam folds, conform folds and overturned folds are found (Fig.1). The sedimentary formations beds overlie the crystalline formations in the areas of northeastern, eastern and southeastern parts. Sedimentary rocks comprised of laterite uplands, sandstone (Tertiary age), an isolated pocket of durry crust (Cretaceous formations) and patches of Gondwana formations. The boundary between crystalline and sedimentary units lies east of Senkipatti and north of Alathur. The major rock types are encountered in the sedimentary areas sandstone, clay, laterites, sands shale, kankar, limestone and fossiliferous limestone etc.,
Well Inventory
Well inventory was conducted by open well observing the well cross sections and lithology of the various bore wells and dug wells information was collected from the well owners and drillers. Well depth, water level below ground level, water level fluctuations and yielding capacities of the individual wells are given in the Table 1.
Geomorphological Studies
The geological, structural, well inventory, hydrological data was integrated with the remote sensing technology information for the preparation of hydro-geomorphological maps (Fig.2)
Structural hills:
Structural hills are the linear or arcuate hills exhibiting definite trend lines and charnockites formations. These hills are structurally controlled with complex folding, faulting, criss-crossed by numerous joints/fractures which facilitate some infiltration and mostly act as run off zones. These are found to be north to northwestern parts of the study areas. The structural trend of the hills ranges in the southwest to northeastern direction with slight deviation towards the western part. The slope of the hills ranges from 2 degree to 4 degree. In this unit groundwater prospects is very poor.
Denudational hills and residual hills:
Denudational hills are marked by sharp to blunt crest lines with rugged tops indicating that the surface run off at the upper reaches of the hills has caused rill erosion. They are mostly exposed to the south to southwest. Denudational hills comprised of charnockites, hornblende biotite gneisses, and anorthosites/ quartzites. These are found to be south-central to southwest. A few pockets of denudational hills exist in the northwestern parts of the study area are mostly covered with forest leaving no common tonal characteristics for their identification. But that could be interpreted from their massive size and dotal to elliptical shape. The rugged topography of this region is due to the erosion of the denudational hills to the plain region, leaving the rock exposed. The groundwater potential is moderate to poor.
Residual hills are the end products of the process of pediplanation, which reduces the original mountain masses into a series of scattered knolls standing on the pediplains (Thornbury, 1990). The hills more resistant formation standing out prominently differential erosion and weathering. It's occurring as isolated patches are found at lower altitudes complex to the denudational hills. In spite of their isolated occurrence, their continuity in a linear or curvilinear fashion gives indication that they are structurally controlled. The shape of the residual hills, controlled by the different lithological composition, distribution and spacing of joints and fractures. In the imageries dark grey tone and coarse texture in block and white images and dark reddish colour in false colour composite with radial drainages pattern (Gupta 1980). Groundwater prospects are beloved to be poor in the areas.